Mars had an oxygen-rich atmosphere four billion years ago – more than a
billion years before Earth, according to a study of rock samples.

Rocks collected by Nasa's Spirit rover from the surface of the Gusev crater were found to contain five times as much nickel as Martian meteorites found on Earth.

This suggests that the surface rocks, which are at least 3.7 billion years old, formed in an oxygen-rich environment while the meteorites, aged between 180 million and 1.4 billion years, did not.

Researchers said the most likely explanation for the difference was due to a process known as subduction, which occurred during an early stage of the planet's history when its atmosphere contained high levels of oxygen.

Oxygen-rich material from the surface would have been "recycled", or drawn into the planet's interior and then spurted back out to the surface during eruptions four billion years ago.

In contrast, the meteorites are younger volcanic rocks from deeper parts of the planet and would have been less affected by subduction, the researchers suggested in the Nature journal.

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Whether the findings have any bearing on the likelihood of past life on Mars is uncertain, because the oxygen could either have been produced by organisms or by a chemical reaction in the atmosphere.

Professor Bernard Wood of the department of earth sciences at Oxford University, who led the study, explained: "What we have shown is that both meteorites and surface volcanic rocks are consistent with similar origins in the deep interior of Mars, but that the surface rocks come from a more oxygen-rich environment, probably caused by recycling of oxygen-rich materials into the interior.

"The implication is that Mars had an oxygen-rich atmosphere at a time about 4000 million years ago, well before the rise of atmospheric oxygen on Earth around 2500 million years ago.

"As oxidation is what gives Mars its distinctive colour it is likely that the 'Red Planet' was wet, warm and rusty billions of years before Earth's atmosphere became oxygen rich."